Summary: The Philosophy Of Seismic Design

CHAPTER NO. 2

The philosophy of seismic design

According to the severity, earthquake can be minor, moderate or strong. These can be categorize on their occurrence; minor occurs frequently, moderate occurs occasionally and strong occurs rarely. The question is, should we design a structure for a rare earthquake that is expected once in thousands of year or should we design for an earthquake that is expected to hit more frequently. To make a building earthquake proof with no damage during the strong ground shaking is too expensive and on the other hand designing for minor earthquakes can be disastrous. So, the philosophy of seismic design is somewhere between this.

Importance of a structure and economic consideration, these are the two main

From building point of view there exist two types of analysis, linear analysis and non-linear analysis.

Linear Analysis

In linear analysis, stress-strain relationship is considered as linear throughout the analysis. Usually linear analysis is not used for the structures with one or more irregularities. Results could be inaccurate if linear analysis is used for highly irregular structures, expect the structure behaves in almost elastic manner to the design loads.

The use of linear procedure is determined by Demand-Capacity Ratios (DCRs) which can be calculated by equation 2.1.

DCR=Q_UD/Q_CE

Where,

Q_UD = Forces due to earthquake and gravity loads

Q_CE = Component’s expected strength

Following is criteria for linear analysis applicability: Linear analysis can be adopted if DCRs < 2.0 for all

V=F_t+∑_(i=1)^n▒F_i

Ft is the concentrated force at the top and should be calculated from

F_t=0.07 T V

Where, T is the time period of the structure. Ft should not be greater than 0.25V and can be taken as zero if T is less than or equal to 0.7 sec.

Remaining part of base shear is distributed throughout the structure’s height by following formula

F_x=((V-F_t)〖 w〗_x h_x)/(∑_(i=1)^n▒〖w_i h_i 〗)

Ref: 1997 Uniform Building Code (UBC) Volume 2: Chapter 16 Structural Design Requirements Section: 1630.2 Response Spectrum Analysis

General

Response spectrum analysis is a dynamic analysis in elastic region which utilizes the maximum dynamic response of all modes of the structure including significant contribution of total response of the structure. Maximum modal responses are determined from response spectrum curve corresponding to modal periods.

Ref: UBC volume